This application is based on German priority application 19930152.2, which is hereby incorporated in its entirety by reference.
This invention relates generally to systems for mixing radio frequency (xe2x80x9cRFxe2x80x9d) signals and more specifically to a four quadrant multiplier with an adjustable linearity range.
Systems for mixing RF signals are known in the art. Typically, mixers are fed two RF signals. One signal is a received, modulated RF signal and the other signal is a local oscillator output RF signal having a fixed reference frequency. Certain types of such mixers are able to reduce the received, modulated RF signal to an intermediate frequency (xe2x80x9cIFxe2x80x9d) signal. To achieve this result, the mixer must be adapted to accept both positive and negative signs for both RF signals. One such mixer is a four-quadrant multiplier, which can also referred to as a Gilbert cell. FIG. 2 shows a Gilbert cell of the type disclosed in U.S. Pat. No. 5,826,182. The Gilbert cell of FIG. 2 has three differential amplifiers: a first differential amplifier T1, T2 and a second differential amplifier T3, T4, respectively connected as a load of a third differential amplifier T7, T8.
The received, modulated RF signal is fed to the third differential amplifier T7, T8. More precisely, the signal is fed to the base terminals of the transistors T7, T8 of the third differential amplifier. The local oscillator output RF signal is fed to the base terminals of the transistors T1, T2 and T3, T4 of the first and second differential amplifiers, respectively. Alternatively, the received, modulated RF signal may be fed to the first and second differential amplifiers and the local oscillator output RF signal may be fed to the third differential amplifier. A current source I for the transistors T7, T8 is provided on the emitter side. The IF signal can then be tapped off at the summing points of the collectors of the cross-connected transistors T1, T2, T3, T4 of the first and second differential amplifiers.
Tietze, Schenk xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d [Semiconductor Circuitry], 9th edition, Springer-Verlag, 1991, page 351 et seq., teaches the connection of the emitters of the transistors T7, T8 to a negative feedback resistor and the provision of respective current sources for each of the transistors T7, T8.
An important characteristic of such a four-quadrant multiplier is its linearity range. The linearity range of a four-quadrant multiplier refers to the voltage range of the input signals in which the multiplier exhibits a largely linear response. The linearity range required for the mixer depends on the nature of the application in which the mixer is being used. It is known that the linearity range of such a mixer may be adjusted externally by using additional resistors to increase the current. A Gilbert cell modified to include such resistors is shown in FIG. 3. The external resistors Rext are provided on the emitter side of the transistors T7, T8. A bias voltage VBIAS1, for setting the operating point is applied to the control terminals of the transistors T7, T8. A bias voltage (not shown) for setting the operating point is similarly applied to the transistors T1, T2, T3, T4 of the first differential amplifier stage.
Certain problems arise when such external resistor circuitry is used to adjust the linearity range of the mixer. First, in modern applications, such as the integration and use of image frequency suppression mixers on integrated circuits, the output of a low noise amplifier (xe2x80x9cLNAxe2x80x9d) is connected directly to the mixer input. Therefore, the reception signal amplified by the LNA no longer leaves the integrated circuit. In such circuits, the emitters of the transistors are not used as the input. Rather, the base terminals of the transistors are used as the input. Second, mixers incorporating such external resistor circuitry cannot be completely integrated with other circuits.
Therefore, there is a need to provide a four-quadrant multiplier with an adjustable linearity range. There is also a need to provide a four-quadrant multiplier with an adjustable linearity range that is adapted to be completely integrated with other circuits.
The present invention provides a four-quadrant multiplier with adjustable linearity. The multiplier comprises first and second differential amplifiers which are respectively connected as a load for a third differential amplifier. Control terminals of the first and second differential amplifiers are fed a first RF signal. Control terminals of the third differential amplifier are fed a second RF signal. In a preferred embodiment, the first RF signal is an RF signal at reference frequency from a local oscillator, and the second RF signal is an amplified, modulated reception signal.
At least one adjustable resistance element is provided between collectors of transistors of the first and second differential amplifiers. The adjustable resistance element may also or alternatively be provided between emitters of transistors of the third differential amplifier. The adjustable resistance element may also be provided between a current source and one of the emitters of the transistors of the third differential amplifier.
For example, the adjustable resistance element may be an adjustable negative feedback resistance element provided between the current source and one of the emitters of the transistors of the third differential amplifier. For another example, the adjustable resistance element may be an adjustable load change resistance element provided between the collectors of the transistors of the first and second differential amplifiers.
Some embodiments comprise both the adjustable negative feedback resistance element and the adjustable load change resistance element. In some of the embodiments, the present invention provides means for the synchronous switching of the adjustable negative feedback resistance element and the adjustable load change resistance element. Such means may comprise, for example, a bus which controls the synchronous switching.
In other embodiments which comprise both the adjustable negative feedback resistance element and the adjustable load change resistance element, one of the adjustable negative feedback resistance element and the adjustable load change resistance element is a p-channel resistor and the other of the adjustable negative feedback resistance element and the adjustable load change resistance element is an n-channel resistor.
Other embodiments comprise a fixed negative feedback resistance element that is not adjustable and that is provided between the current source and the emitters of the transistors of the third differential amplifier. The resistance of the adjustable resistance element may be changed continuously at least in a predetermined range. This is achieved, for example, by using with a transistor.
The present invention additionally provides a receiver for modulated RF signals comprising the four-quadrant multiplier in any of the embodiments described above, and wherein an amplified, modulated RF signal is fed to the control inputs of the first and second differential amplifiers through an LNA and a local oscillator output RF signal having a fixed reference frequency FREF is fed to the control inputs of the third differential amplifier. Alternately, the amplified, modulated RF signal is fed to the control inputs of the third differential amplifier through an LNA and the local oscillator output RF signal having a fixed reference frequency FREF is fed to the control inputs of the first and second differential amplifiers.
Further objects, features and properties of the present invention will become apparent from the detailed description of the preferred embodiment that follows with reference to the accompanying drawings.